The invention relates generally to mobile communications. In particular, the invention relates to methods, computer programs, apparatuses and radio network nodes for dynamic uplink/downlink configuration for time division duplex.
Long Term Evolution (LTE) was introduced in release 8 of 3rd Generation Partnership Project (3GPP) which is a specification for 3rd generation mobile communication systems. LTE is a technique for mobile data transmission that aims to increase data transmission rates and decrease delays, among other things. LTE uses orthogonal frequency division multiple access (OFDMA) as its multiple access method in the downlink. The uplink uses single-carrier frequency division multiple access (SD-FDMA). 3GPP release introduced a next version of LTE, named LTE Advanced, fulfilling 4th generation system requirements.
Both LTE and LTE Advanced may utilize a technique called time division duplex (TDD) for separating the transmission directions from the user to the base station and back. In TDD mode, the downlink and the uplink are on the same frequency and the separation occurs in the time domain, so that each direction in a call is assigned to specific timeslots.
Herein, the term “downlink” (DL) is used to refer to the link from the base station to the mobile device or user equipment, and the term “uplink” (UL) is used to refer to the link from the mobile device or user equipment to the base station.
FIG. 4 illustrates the frame structure for LTE TDD. The uplink and downlink for LTE TDD are divided into radio frames 400, each of which is 10 ms in length. The radio frame 400 consists of two half-frames 411, 412, both of which are 5 ms long. The first half-frame 411 is further split into five subframes 420-424, each 1 ms long. Similarly, the second half-frame 412 is further split into five subframes 425-429, each 1 ms long. Subframes 420, 422-425, and 427-429 are reserved for either downlink or uplink data, whereas subframes 421 and 426 are so called “special” subframes that include three special fields: downlink pilot time slot (DwPTS), guard period (GP) and uplink pilot time slot (UpPTS). However, as discussed below, in some configurations subframe 426 may also be reserved for downlink data, with the subframe 421 being the only special subframe. All non-special subframes consist of two time slots, both 0.5 ms long.
TDD allows asymmetry of the uplink and downlink data rates, i.e. as the amount of uplink or downlink data increases, more communication capacity can be allocated, and as the traffic load becomes lighter, capacity can be taken away.
This asymmetry is implemented via seven different semi-static uplink-downlink configurations, illustrated below in Table 1:
TABLE 1Uplink/downlinkSubframe numberconfiguration01234567890DSUUUDSUUU1DSUUDDSUUD2DSUDDDSUDD3DSUUUDDDDD4DSUUDDDDDD5DSUDDDDDDD6DSUUUDSUUD
In Table 1, “D” indicates that downlink data is transmitted in this subframe, “U” indicates that uplink data is transmitted in this subframe, and “S” indicates that the special fields DwPTS, GP and UpPTS are transmitted in this subframe. As can be seen, the seven different uplink/downlink configurations 0-6 contain different ratios of uplink and downlink data, and allow asymmetric uplink and downlink data rates.
Furthermore, in all seven configurations 0-6 subframes 0 and 5 are always for downlink, subframe 1 is always a special subframe, subframe 2 is always for uplink, and subframe 6 is a special subframe or for downlink. In other words, no matter which uplink-downlink configuration is applied, there are always subframes with fixed link direction. Herein, such subframes with fixed link direction are referred to as fixed subframes. Subframes with non-fixed link direction are herein referred to as non-fixed subframes.
The above prior art uplink-downlink configurations can provide between 40% and 90% DL subframes. The current mechanism for changing from one uplink-downlink configuration to another is based on a system information exchange procedure. However, since system information is sent at the interval of at least 640 ms, it cannot provide dynamic TDD configuration to adapt to an instantaneous traffic situation, leading to inefficient resource utilization, especially in cells with a small number of users where the traffic situation changes more frequently.
Furthermore, in LTE TDD systems, many operations at both evolved Node B (eNB) and user equipment (UE) sides depend on the semi-static TDD configuration. These operations include e.g. radio resource management (RRM) measurements, channel quality information (CQI) measurements, channel estimations, physical downlink control channel (PDCCH) detections, and hybrid automatic repeat request (HARQ) timings.
The UE firstly needs to read the system information to find out the TDD UL/DL configuration in its current cell. Then it knows which subframe to monitor for measurement, for CQI measure and report, for time domain filtering to get channel estimation, for PDCCH detection, or for DL/UL ACK/NACK feedback. For example, in the ACK/NACK multiplexing scheme, the feedback values of b(0),b(1) and the ACK/NACK resource nPUCCH(1) are generated by channel selection according to Tables 10.1-2, 10.1-3, and 10.1-4 in 3GPP TS 36.213 V9.0.1 specification (December 2009) for M=2, 3, and 4, respectively. Also, the UE needs firstly get the TDD UL/DL configuration so that it knows the correct table to use. Otherwise, there will be a detection error at the eNB side. After that, correct operation depends on the correct understanding of the signaling indicating the TDD UL/DL configuration.
Prior art also includes indicating the TDD UL/DL configuration implicitly via a scheduling grant. However, the problem with this is that if there is no scheduling grant for a given UE, the UE will never know the link direction of the non-fixed subframes. Therefore, it cannot use these subframes for RSM measurement, CQI measurement, or filtering for channel estimation. In practice, the CQI in the non-fixed subframes may be quite different from that in the fixed subframes, due to e.g. different interference levels. Thus, enabling UE's CQI measurement in non-fixed subframes may provide the network side relevant information for better resource scheduling. Moreover, the UE has to monitor the non-fixed subframes for PDCCH before knowing if it is DL or UL, and this increases the UE's power consumption. Yet another problem is on the HARQ timing: if there is no scheduling grant for a given non-fixed subframe, the UE will not be aware of the real TDD UL/DL configuration. Therefore, it cannot use the TDD UL/DL configuration dependent. HARQ timing as specified in Release 10. A solution could be to restrict the HARQ feedback to a fixed subframe, but this would lead to increased HARQ delay.
Therefore, an object of the present invention is to alleviate the problems described above and to introduce a solution that allows dynamic TDD UL/DL configuration that is able to adapt to an instantaneous traffic situation.